The objective of this project is to elucidate the energetic consequences of Ca 2 ion cycling during skeletal muscle contraction and to determine the effects on Ca 2 ion cycling of various experimental interventions including: muscle length and rapid changes of muscle length, stimulus frequency, and chemical agents. The following rationale will be employed. It has been shown that the net heat produced during muscle contraction is proportional to the measured splitting of ATP. This seems true irrespective of the type of contraction. Also, if a muscle is stretched to the point of vanishing overlap of the thick and thin filaments and then stimulated, the net heat liberated can be attributed to ATP splitting. It seems likely that this ATP is split to resequester Ca 2 ion by the sarcoplasmic reticulum. Assuming that all the Ca 2 ion released must be actively reaccumulated again, then the net heat production from stretched muscles indirectly gives a measure of the amount of Ca 2 ion released with stimulation. Thus alterations in the time course and net heat liberation attributed to Ca 2 ion cycling reflect alterations in Ca 2 ion release and reaccumulation. Heat production during isometric contractions of isolated frog semitendinosus muscles will be monitored with sensitive thermopiles at 0 to 15 degrees C.